Pile foundations have been used for major structures for a very long time. It has also been very difficult to determine when the bearing capacity of the driven pile is sufficient to support the intended load. Ever since pile foundations were introduced, engineers have calculated the bearing capacity by empirical formulas and have used the costly dead load test to verify the results. In a dead load test a test pile is loaded incrementally over a period of time to twice its intended load and then the load is gradually removed. About thirty empirical formulas have been devised, all equating the bearing capacity to the hammer energy, divided by the set, plus a coefficient for the compression of the pile, drive cap and soil. The set is the net movement of a pile after it is struck with a hammer. Some formulas include other arbitrary factors such as: the coefficient of restitution; the weight of the pile divided by the ram weight; and the soil type. In 1957, Mr. E. A. Smith applied Timoshenko's elastic wave equations for rod-like objects to a pile without skin friction. He proved that the two most popular bearing formulas, The Engineer News and The Hiley, were flawed in many respects. In the early sixties, Michigan State University performed extensive tests using various hammers and piles. All of the proposed formulas yielded various results, but none matched or correlated acceptably with the dead load test. Neither Smith or Michigan suggested a revised formula nor developed a new one. Nevertheless, in the seventies, the elastic wave equation formulas were put to commercial use and were recommended by many foundation authorities as the best possible solution to this very difficult problem. They too have found the results at variance with the dead load test and have had to add reconciliation factors to make them agree. I have made a number of attempts to instrument a pile so that the bearing capacity of the pile can be determined while it is being driven. U.S. Pat. Nos. 3,817,091; 3,897,835 and 3,931,729 document these efforts; testing indicated the need for faster more sophisticated recording instruments.
In practice, engineers responsible for providing a sound foundation continue to doubt the reliability and accuracy of these arbitrary factors and adjustments. In the absence of a viable alternative, the costly dead load test is generally performed anyway.